The present invention is directed to a process and a device for measuring the lengthwise and crosswise displacement of a paper or board web travelling through a rotary multi-color printing press, which device is being put into action with every color. Such a device allows a control of the required vertical and/or lateral cylinder displacement downstream with a view to insure a precise juxtaposition and registration of the successive basic color prints to produce an image.
Numerous presently known devices make use of several marks which are successively printed jointly with the various colors. These marks pass under the photoelectric detectors to cause them to generate signals with a combination allowing to determine the amplitude from the displacement value.
French Patent Document 1,470,034 describes a device having five photoelectric detectors arranged above and along a line extending crosswise to the web for scanning a mark having a shape of a line which extends crosswise to the printed web and is situated in a printless area between two prints. Operating jointly with a tachometer, the electric circuit of the device is essentially foreseen for checking whether the detectors are influenced by the characteristic mark in the center of a sufficiently broad printless area and not by other printed sections.
French Patent Document 2 362 451 discloses a device which comprises two photoelectric detectors adjacently arranged along the web travelling direction in such a way that the detection of a mark passing under these detectors may be achieved with sufficient precision. To this aim, a switching signal is emitted if the difference between the intensity of the signals read by the two detectors reaches a preset minimum value or rate. With this device, the mark dimension in the travelling direction should preferably be higher than the scanning field of the two detectors.
U.S. Pat. No. 4,450,766, whose disclosure is incorporated herein by reference thereto, describes a device for checking the multi-color juxtaposition or registration on the basis of marks with the shape of a right-angled triangle of which the two sides adjacent to the right angle extend parallel both to the travelling direction and to the webs width. A comparison of the time lag of one signal with regard to the other signal as a result from two different colors provides the value of the lengthwise displacement. A comparison of the width of the signal emitted by a detector with a reference value allows the obtaining of the value of the crosswise displacement with the corresponding color. However, a reduction of the width of such a mark in the web travelling direction will also cause a reduction of the declivity of the hypotenuse with regard to the orientation of the width which, in turn, will also considerably reduce possible variations originating from the signal width. Considering the minute error of measurement caused by the photoelectric detectors, the crosswise measurements quickly will take on a negligible value.
As has become conspicuous, the shortcomings of the abovedescribed devices is inherent in obligatorily respecting the minimal rates of the mark requested to insure proper operation and to the detriment of the area available for the print.